1. Field of the Invention
This invention relates to a voice guard circuit of the type used in tone receivers used in telecommunications, and more particularly to such a circuit of special utility for use in tone receivers for decoding Dual Tone Multiple Frequency (DTMF) signals.
2. Description of the Prior Art
A tone receiver is an electronic device used in telecommunication systems to convert a Dual Tone Multiple Frequency (DTMF) signal into a multiple digit binary code. The format of a DTMF signal consists of high and low signal bands (679-941 Hz. and 1209-1633 Hz.). Each band carries a selected one tone signal signal of a predetermined set of four tones unique to the respective band. That is, there is simultaneous presence of two tones, with one in each band, and with each such one tone in each band being a selected one of four possible tones for the band. The DTMF signal system is sometimes referred to as the "2-of-8" system.
A voice guard circuit operates in conjunction with such a tone receiver to insure that neither voice signals (the frequency range of the human voice spans the bands of DTMF signals) nor other cross modulation products can inadvertently simulate the DTMF signal.
It is desirable to provide tone receivers having large dynamic ranges. Known prior art forms of voice guard circuits for tone receivers have employed the approach of recognition of invalid signal conditions by reference to fixed reference levels or thresholds. These prior art approaches have been a limiting factor in the design of tone receivers having large dynamic ranges.
Accordingly, an object of the present invention is to provide a voice guard circuit for use with a tone receiver which is not significantly limited by signal dynamic range of the tone receiver.
Another object of the invention is to provide a voice guard circuit which reliably rejects voice signals.